Among drug solution injectors for continually injecting drug solutions such as such as an anesthetic, an analgesic, an anticancer agent, an antibiotic or the like into the body, drug solution injectors of a type that requires no power supply have widely been used in a home-care setting or in an ambulatory setting because of their high portability to administer an anticancer agent or a pain relief.
The drug solution injectors of such a type generally comprises a drug solution reservoir having a chamber for storing a drug solution therein and a pressurization mechanism for continually discharging the drug solution in the chamber to the outside, and an injection line connected to the drug solution reservoir to inject the drug solution into the body. An example of the drug solution reservoir widely used includes balloons of an elastic material. The balloons used as a drug solution reservoir is filled with a drug solution and then allowed to use contraction stress of the inflated balloon to discharge the drug solution to the outside.
The drug solution injector with such a balloon as a drug solution reservoir is generally provided with a housing for holding a balloon to protect the balloon. It has been known to mark scales corresponding to inflation and deflation of the balloon on the housing (cf. FIG. 1, scale 111). The scales make it possible to give an approximate injection volume of the drug solution (a decreased volume of the drug solution in the balloon) or an injection rate of the drug solution (cf. Patent documents 1-4). However, the above scale has been marked to give an index of the injection volume of the drug solution, thus making it difficult to determine an accurate injection volume of the drug solution because of deviations in inflated shape of the balloons at the time of balloon molding.    Patent document 1: JP H03-140163A    Patent document 2: JP H05-115542A    Patent document 3: JP H06-154320A    Patent document 4: JP H07-116250A